1. Field of the Art
This invention relates to an electronic endoscope suitable for use in medical and industrial fields, and more particularly to an electronic endoscope capable of producing a vivid freeze-picture image of a subject under observation through a solid-state image sensor driven by a sequential color scan system.
2. Description of the Prior Art
As well known in the art, electronic endoscopes have a solid-state image sensor like CCD mounted at the tip end of an insert or catheter member to be introduced, for example, into an intracavitary portion of interest, and, for reducing the diameter of the insert member, generally employ a single-element solid-state image sensor. From the standpoint of improving the resolution, the solid-state image sensor element is driven, for example, by a sequential color scan system to sequentially obtain picture data of R (red), G (green) and B (blue), and these tricolor picture data are processed to produce a color picture image of the subject. In addition to observation of motion video images of a subject, it is often the case that arrangements are made to produce still picture images of a subject for display on a monitor or for hard-copying or for recording on an external memory medium such as video tape, magnetic disk, optical disk and so forth. In order to obtain a still picture image or a freeze-image, electronic endoscopes are usually provided with a freeze control mechanism, including a freeze button which is mounted on a manual operating section of each endoscope for manipulation by an operator's finger when it is desired to freeze the picture image.
In this regard, in order to obtain a complete picture image of a subject, it is necessary to mix a plural number of sequentially varying picture data (the RGB tricolor picture data in case of field picture, and the six picture data consisting of RGB picture data of odd- and even-number fields in case of frame picture). Aside from the display of motion-picture images, the production of a still picture image has a problem that a relative movement between a subject and a solid-state image sensor element on the insert member of the endoscope often results in an obscure picture image of low resolution due to deviations of color signals. Therefore, in order to pick up a sharp freeze-picture image free of color deviations, it has been the general practice to detect motions of a subject on the basis of the picture images sequentially picked up through the solid-state image sensor element, and to freeze the picture image only when a still state of the subject is detected by a motion sensor circuit, without immediately responding to a freeze control signal. More specifically, in processing signals from a solid-state image sensor, the obtained picture data are fed to a motion sensor circuit before A/D conversion, thereby to detect a motion of a subject by comparing the picture data of successive fields (or frames), e.g., detecting a difference in contour or other picture elements by comparing a real-time signal of R color data with preceding R color data, namely, with a corresponding R signal of three fields before. When the subject is found to be in a still state or to be only in a slight motion which would not cause deviations of color signal, the data renewal of picture memory circuit is inhibited and the picture data of this state is displayed on a monitor as a freeze-picture image.
As explained above, it has been difficult to obtain a freeze-picture image immediately in response to manipulation of a freeze control button, which only starts the operation of the motion detection circuit. Namely, a freeze-picture image is displayed at a time point when a subject is found to be in a still state after manipulation of the freeze button, in some cases letting a relatively long time lapse before freezing the image. As a result, it is often the case that the resulting freeze-picture image is different from an originally intended one, failing to show the condition of particular locality of interest at the proper timing and, due to inferior response of the freeze-picture control, necessitating repetition the freeze operation until an optimum or satisfactory freezeimage is obtained.